FIELD OF THE INVENTION
The invention relates to a circuit configuration for sensing when a critical temperature of a component has been exceeded. At least one sense transistor with a temperature-dependent current/voltage characteristic is thereby thermally connected to the component. A current source is connected in series with the sense transistor.
A circuit configuration of that type is disclosed, for example, in European published application EP 0 341 482 A1.
The principle of a temperature sensor of the above-mentioned type is best explained with reference to FIG. 1 of the drawing: The circuit in FIG. 1 has a bipolar transistor T.sub.1 with a current source SQ connected in series. The bipolar transistor T.sub.1 is thermally connected to the semiconductor component, e.g. a power MOSFET or a power IC, which is to be monitored for overheating. The current source SQ may be a MOS semiconductor component, for example using an n-channel depletion-type transistor. The series circuit comprising the current source SQ and the bipolar transistor T.sub.1 is connected between the terminals of a first and a second supply potential V.sub.CC, GND. The base of the transistor T.sub.1 is connected to a reference voltage source VR.
The current source SQ supplies the bipolar transistor T.sub.1 with a constant current I.sub.B. At the same time, a constant base voltage is impressed on the base terminal of the transistor T.sub.1 by the reference voltage source VR. At low temperatures, the bipolar transistor T.sub.1 is off and its collector potential V.sub.Out corresponds to the value of the operating voltage. With rising temperature and constant base-emitter voltage, if the ability of the bipolar transistor to draw collector currents exceeds the ability of the current source SQ to supply constant current, the collector potential switches to LOW at the output of the temperature sensor configuration. By appropriately selecting the reference voltage V.sub.R at the base, the turn-off temperature can be selected to suit specific requirements.
Such a temperature sensor configuration must reliably register the temperature of a semiconductor component HLB. If the semiconductor component HLB overheats, then the configuration must produce a signal that switches off the semiconductor component HLB. If this does not happen, the semiconductor component HLB may be destroyed.
If a pn-junction is reverse-biased, the applied reverse voltage causes pairs of electron holes, which are produced throughout the diode by thermal generation, to be separated at the edges of the space charge zone, and the minority carriers to be pulled away via the space charge zone. Recombination drops off in the face of thermal generation. If the temperature rises, generation also increases, more minority carriers pass through the space charge zone and the current rises exponentially.
The parasitic depletion layer leakage current is negligible at room temperature. However, the parasitic leakage current reaches dangerously high values at higher temperatures, usually at temperatures above 150.degree. C. For example, the parasitic collector-substrate current I.sub.CS1 in a conventional npn-bipolar transistor at 300.degree. C. is as much as about 30 mA. The parasitic collector-base current I.sub.CB1 also reaches undesirable levels.
Reliably registering the temperature of a semiconductor component is possible only to a limited extent, however. Particularly at temperatures above 150.degree. C., parasitic depletion layer leakage currents take effect to an increasing degree. These parasitic leakage currents significantly impair the operation of the temperature sensor and can even render it inoperative. Particularly at very high temperatures, this can even result in the destruction of the semiconductor component HLB.